The invention relates generally to interferometers and more specifically concerns a dual differential interferometer that measures both the amplitude and orientation of propagating, broadband ultrasonic surface acoustic waves (SAW).
Two monochromatic laser beams interfere according to their relative phase. The interference pattern produced is an intensity distribution consisting of light and dark regions referred to as fringes. Since the phase transverses 2.pi. radians in each wavelength, the relative phase for two beams derived from the same source can be considered as their relative pathlength difference. Points in the image plane with pathlength differences equal to integer multiples of wavelength constructively interfere. Half wavelength path differences destructively interfere. The changing of pathlength in time of one beam relative to the other causes a synchronous translation of the fringes. A displacement of one wavelength produces a shift of one fringe position in the interference pattern. If both beam paths change simultaneously by the same distance, there is no fringe translation. The motion of the fringes can be monitored by an optical detector.
For surface acoustic wave measurements, two monochromatic beams are focused on the surface of a specimen one-half the SAW wavelength apart. The reflections from the incident beams are then combined to form a fringe pattern and is called a differential interferometer. In the relaxed state, the fringe pattern will remain fixed. Fringe translations occur when a SAW generated on the specimen interacts with the incident beams. The interaction is due to the modulation of the specimen surface by the SAW on which the beams are reflected. This modulation varies in time the pathlengths of each beam. The incident beams placed one-half wavelength of the SAW apart have maximum differential pathlength modulation. If placed exactly a wavelength apart, the displacement in relative pathlength would be zero and no fringe shifts would occur. Also, a translation of the specimen would not produce fringe shifts.
The two beam differential interferometer has several limitations. The modulation of the focused spots is dependent not only on the amplitude of the SAW but also on the angular orientation of the SAW propagation with respect to a chord passing through the two spots. The detected amplitude of a SAW is thus the component of surface wave motion along the direction parallel to the chord. Accordingly, SAWs of unequal amplitude and different angular orientations may appear the same to the two-beam interferometer. In fact, a SAW propagating at a 90.degree. incidence to the reference chord would ideally generate no detectable sign of its presence.
It is an object of this invention to provide an interferometer that measures both the amplitude and orientation of propagating, broadband SAWs.
Another object of this invention is to provide an interferometer for measuring the amplitude of progagating broadband SAWs without regard to the orientation of the propagating waves.
A further object of this invention is to provide means for increasing the sensitivity of optical detectors used in two-beam differential interferometers.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.